Method of fabricating a photo-device

ABSTRACT

A method of fabricating a photo-device includes the steps of forming an optically nontransparent, electrically conductive layer on the surface of a photoelectric conversion substrate, and transferring a portion of the optically nontransparent, electrically conductive layer using a scanning probe process apparatus to form, over an optical window, a light-permeable protective insulation structure of a region of the optical window on the substrate surface beneath the light-permeable protective insulation structure.

This is a continuation of application Ser. No. 08/590,345 filed on Jan.23, 1996 now U.S. Pat. No. 5,661,328.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved photo-receiving device forconverting light signals to electrical signals, as required in suchfields as communications and information processing, and to an improvedmethod of fabricating a photo-device, which also means a light emittingdevice for converting electrical signals to light signals.

2. Description of the Prior Art

The term photoelectric conversion as used herein refers to the functionof converting a light signal to an electrical signal in aphoto-receiving device, and also to the function of converting anelectrical signal to a light signal in a light emitting device. FIG. 8illustrates the structure of a conventional photo-receiving device 50 asdescribed in the IEEE Journal of Quantum Electronics (Vol. 28, pp.2358-2368, 1992) for converting light signals to electrical signals at arelatively high speed. As shown, the photo-receiving device 50 comprisesa substrate 11 that constitutes the photoelectric conversion portion(which can also be referred to as a light-absorbing portion, as this isa photo-receiving device), and a pair of opposed electrode portions 51,each in the form of a thin film of metal, formed on the surface of thesubstrate 11. The exposed surface of the substrate 11 between theelectrodes 51 forms an optical window 13 for the entry of the lightI_(P) to be detected.

When light I_(P) impinges on the optical window 13 while an appropriatevoltage is being applied to the pair of electrodes 51, excited carriers(electrons and holes) are generated in the substrate 11. Holes, shown inthe drawing as blank circles, are drawn to the electrode 51 with therelatively negative (-) potential, and electrons, shown as solidcircles, are drawn to the electrode 51 with the relatively positive (+)potential, setting up a flow of a photoelectric current (photo detectioncurrent) which takes place via the electrodes 51, whereby the incidenceof the light I_(P) is detected.

With this type of conventional photo-receiving device 50 of FIG. 8,which is generally referred to as a MSM (metal/semiconductor/metal)device, the smaller the width W of the optical window 13, that is, thesmaller the distance between the electrodes 51, the faster the operatingspeed of the device is, and raising the applied voltage increases devicespeed and sensitivity. Also, making the width W of the optical window 13no larger than the wavelength of the light I_(P) to be detected impartsan evanescent field to the light I_(P) impinging on the light-absorbingsubstrate 11 and causes the incident light I_(P) to be absorbed in thevicinity of the surface of the substrate 11. At the same time, as thefield strength set up by the electrodes 51 is higher at the surface ofthe substrate 11 than in the interior, the excited carriers generated inthe vicinity of the surface of the substrate 11 are drawn rapidly to theelectrodes 51, enabling higher speed operation to be achieved and theaffect of carrier recombination to be reduced.

In the case of the photo-receiving device 50 of FIG. 8, reducing thewidth W between the electrodes 51 to around 300 nm by means of electronbeam lithography, an existing fine pattern process technology, resultedin a pulse-response full width at half maximum output of 870 fs, whichis quite a high speed compared to other photo-receiving devices.However, it is difficult to achieve higher speeds, for the followingreasons.

A first problem is that, since the optical window 13 on the surface ofthe substrate 11 between the electrodes 51 is exposed, applying a highervoltage across the electrodes 51 giving rise to creeping discharge alongthe exposed surface of the optical window 13 and air-gap discharge,rendering the device unusable. That is, if the width W between theelectrodes 51 is reduced beyond a certain limit, even a low voltagecauses a dielectric breakdown. On the other hand, even in cases wherethe width W of the incident light window 13 can be increased, within thelimitation that it does not exceed the wavelength of the incident lightI_(P), there are major constraints on the voltage that can be applied. Asecond problem is that of the limits of the process technology. Evenwith existing electron beam lithography, a relatively high precisionfine pattern process technology, an electrode gap cannot really beprecision-fabricated to a width W of 100 nm or less, and even 300 nm orless is quite difficult.

A conventional method of resolving the first problem is to cover, orbury, the exposed surface portion constituting the optical window 13between the electrodes 51. This will now be described, with reference toFIG. 9. The following description is not limited to photo-receivingdevices, being also applicable to devices, such as light emittingdevices, having a light emitting area defined in the form of an opticalwindow. Thus, reference numeral 52 is used to denote the photo-deviceshown in FIG. 9, the light-absorbing portion 11 of FIG. 8 is thephotoelectric conversion portion in the general meaning of the term, andthe term metallic film electrodes 51 is encompassed by the termoptically nontransparent conductive film (electrodes) 12. To fabricatethe photo-device 52, existing lithographic technology is used to removea prescribed region of an optically nontransparent conductive film 12formed on photoelectric conversion portion 11, thereby exposing adefined portion that forms an optical window 13.

Sputtering or another such vapor deposition technique is then used toform an optically transparent protective insulation layer 53 over theoptical window 13. However, sputtering and other such vacuum vapordeposition apparatuses are costly, so there is no objection to achievingthe required result by other means. Also, while the opticallynontransparent conductive film 12 in which the optical window 13 regionis formed (defined) and the insulation layer 53 to protect the opticalwindow 13 are formed using separate, processes, device fabrication canbe simplified by effecting both processes in one step. As described,even with the relatively high patterning precision provided by atechnology such as electron beam lithography, the minimum width W of theoptical window 13 that can be formed in the optically nontransparentconductive film 12 is in the order of 300 nm.

Non-uniformities in the thickness of the surface deposition film givesrise to variation in device characteristics, and high frequencycharacteristics can be degraded by dielectric deposits. When a very fineoptical window is used, forming a high-quality insulative film over thewindow that has high dielectric resistance is difficult.

An object of the present invention is to provide a high speedphoto-receiving device in which constraints relating to the width W ofthe optical window between electrodes, and to the applied voltage, arereduced.

Another object of the present invention is to provide a photo-receivingdevice that is more highly functional and multifunctional thanconventional photo-receiving devices.

Yet another object of the present invention is to provide a method offabricating a photo-device wherein an optical window region and aprotective layer over the optical window surface can be formed in onestep.

A further object of the present invention is to provide a method offabricating a photo-device that enables the above-described drawbacks tobe resolved or alleviated and an optical window to be formed to have asmaller width than that of conventional photo-devices.

SUMMARY OF THE INVENTION

To attain the above objects, the present invention provides aphoto-receiving device, comprising a light-absorbing substrate, a pairof electrodes disposed on a surface of the light-absorbing substrate, anoptical window through which light to be detected enters, said opticalwindow being an exposed surface portion of the substrate between theelectrodes, and an optical guide structure provided on the exposedsurface portion of the substrate between the electrodes, said opticalguide being permeable to the light to be detected, having a width thatdoes not exceed a wavelength of the light to be detected, and exhibitinga higher resistance than that of the substrate.

With this configuration, the electrodes can be brought closer togetherthan in a conventional configuration without risk of dielectricbreakdown between the electrodes. This means a stronger electrical fieldcan be applied, which increases the speed of the photocarriers andreduces the effect of carrier recombination, resulting in aphoto-receiving device that exhibits high speed as well as highsensitivity. Moreover, the optical guide also serves as a protectivelayer over the exposed surface portion of the substrate that constitutesthe optical window. This eliminates the need to separately form aprotective layer, thereby simplifying the fabrication process andreducing fabrication cost.

The present invention also provides a photo-receiving device in whichthe optical guide is formed of an insulating material. The insulator maybe an oxide of the thin film material used to constitute the electrodes.

The invention also provides a photo-receiving device in which theoptical guide may be formed of a semiconductor material, provided thesemiconductor material exhibits higher resistance than thelight-absorbing substrate.

In another embodiment, the invention also provides a photo-receivingdevice in which the optical guide is formed of materials havingdifferent properties.

The invention also provides a photo-receiving device in which opticalguide properties such as refractive index, absorption coefficient,polarization direction and the like can be varied. In such anarrangement, control electrodes can be included to apply voltage foreffecting the adjustment of the optical properties of the optical guide.At least one of the control electrodes may be constituted by one of theabove described pair of electrodes. A semiconductor superlatticestructure or Fabry-Perot resonator are representative examples of thetype of arrangement that may be used to vary the optical properties ofthe optical guide.

In accordance with another embodiment, the present invention alsoprovides a photo-receiving device in which the insulation creepagedistance between the electrodes is increased by making the thickness ofthe optical guide greater than the thickness of the electrodes.

In accordance with a further embodiment, the photo-receiving device hasa plurality of optical guides arranged in parallel between the pair ofelectrodes, and means each provided between adjacent optical guides soas not to transmit the light to be detected.

The method of fabricating a photo-device according to the presentinvention comprises a step of forming an optically nontransparent,electrically conductive layer on a surface of a photoelectric conversionsubstrate, and a step whereby a portion of the optically nontransparent,electrically conductive layer is transformed to form over the opticalwindow a light-permeable protective insulation structure of a prescribedwidth and length, and at the same time define an optical window regionon the substrate surface beneath the protective insulation structure.

The optically nontransparent, electrically conductive layer may be of ametal or an alloy, or a semiconductor (including semi-insulators), ormay include a semiconductor superlattice structure.

In this way, a photo-device is fabricated to have a prescribed area on asurface of the photo-receiving or photo-emitting photoelectricconversion substrate forming an optical window of a prescribed width viawhich light is received or emitted. When it is a photo-device thatrequires that an optically nontransparent, electrically conductive layerbe formed at each side of the optical window, the defined optical windowregion and an insulation structure for protecting the optical window canbe formed in a single step. This is highly efficient and eliminates theneed to use a costly vacuum vapor deposition apparatus to form theprotective layer over the entire surface.

In one embodiment of the method of fabricating a photo-device, oxidationis the process used to transform the optically nontransparent,electrically conductive layer. This oxidation can be effected using ascanning probe apparatus.

With this method, a fine optical window having a width that does notexceed the wavelength of the light to be detected, and a high-quality,highly dielectrically-resistant protective insulative structure for theoptical window can both be formed by the same step. The result is thatcarrier speed is increased by applying a high field strength while theeffect of carrier recombination is decreased, thus providing aphoto-device that is faster and more sensitive.

The method for fabricating a photo-device according to this inventionalso comprises transforming multiple mutually separated portions of theoptically nontransparent, electrically conductive layer.

The present invention also provides a method of fabricating aphoto-device in which the photoelectric conversion substrate is of alight-absorbing material having a light receiving function, anon-transformed portion of the optically nontransparent, electricallyconductive layer functions as an electrode to apply a voltage to thelight-absorbing substrate, and the protective insulation structurefunctions as an optical guide that directs light to the substrate.

The present invention also provides a method of fabricating aphoto-device whereby the thickness of the protective insulationstructure preferably is thicker than the thickness of thenon-transformed portions of the optically nontransparent, electricallyconductive layer at each side, and when said portions of the layerremaining at each side are utilized as a pair of electrodes, theinsulation creepage distance between the electrodes is increased,thereby increasing the resistance to dielectric breakdown.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and followingdetailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a method of fabricating the photo-device according toan embodiment of the invention;

FIG. 2 shows the general configuration of an embodiment of thephoto-receiving device of the invention;

FIG. 3 illustrates steps used in the fabrication of an MSMphoto-receiving device in accordance with the method of fabricating aphoto-device of the invention;

FIG. 4 is a diagram of a system used to measure a photo-receiving devicefabricated by the steps of FIG. 3;

FIG. 5 shows the measurement results obtained with the fabricatedphoto-receiving device;

FIG. 6 shows the configuration of another photo-receiving devicefabricated according to the method of the invention;

FIG. 7 shows the configuration of another photo-receiving devicefabricated according to the method of the invention;

FIG. 8 shows the arrangement of a conventional MSM photo-receivingdevice; and

FIG. 9 illustrates a conventional protective structure for an opticalwindow.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an example of basic steps or processes used to fabricate aphoto-device according to the method of the invention. In this and otherembodiments, parts corresponding to parts in the conventional devices 50and 52 of FIGS. 8 and 9, already described, are given the same referencenumerals.

FIG. 1 (A) shows a conductive, optically nontransparent film layer 12formed on a surface of a light receiving or light emitting photoelectricconversion substrate 11. The internal constitution of the substrate 11is not specifically defined by the invention, and may be comprised inaccordance with existing device structural principles. If the device isformed as a conventional MSM photo-receiving device, as described in theforegoing, the substrate 11 will function as a light-absorber, and interms of material, will be formed of a bulk semiconductor such as GaAs.The film 12 may be formed of a suitable metal (including alloys) orsemiconductor, including a semiconductor having a superlatticestructure. In broad terms, a material that is at least non-permeable, orhas very low permeability, to the light to be detected, in the case of aphoto-receiving device, and to the emitted light, in the case of alight-emitting device, and has significant electrical conductivity, canbe regarded as being applicable for the purpose of the invention.

Starting from the structure shown in FIG. 1 (A), the properties of aregion of the optically nontransparent layer 12 corresponding to aregion 13' in which an optical window 13 will ultimately be formed aretransformed to form an insulation structure that is light-permeable(that is, transparent or semitransparent to light that is to be detectedor transmitted, as the case may be). Preferably this is done by using ascanning probe apparatus to oxidize the region concerned. A scanningtunneling microscope (STM) and an atomic force microscope (AFM) areexamples of apparatuses that can be used for this. In the case of anSTM, for example, the tip of the probe P is brought close to the film 12and a power source V is used to apply a high-voltage electrical fieldacross the space between the probe tip and the film 12 while at the sametime the probe P is scanned in a direction S. This oxidizes the film 12along the line of the scan, forming an insulation line 15' that islight-permeable. Using this method, the present inventors were able toform a titanium oxide line with a minimum width of 18 nm by oxidizing atitanium layer deposited on a GaAs substrate. The same result was alsoobtained with an AFM. While not directly related to this invention, withan AFM the target film does not have to be electrically conductive.

When the line 15' has been formed to the prescribed length, alight-permeable protective insulation structure 15 having a prescribedwidth and length is formed to protect the optical window, and the sameformation process is used to define a region for the optical window 13on the substrate surface beneath the structure 15, as shown in FIG. 1(C). This simplifies the fabrication process by eliminating the need forseparately forming a film over the entire surface for an optical windowprotective layer after first forming the optical window, such as isrequired in the case of the conventional device shown in FIG. 9.

With a photo-device 10 fabricated according to this invention, moreover,the remaining nontransformed portions of the optically nontransparentconductive film 12 at each side of the protective insulation structure15 can be used as electrodes. Since in such a case the insulationstructure 15 is disposed between the electrodes 12, the insulationcreepage distance is increased, enhancing the resistance to dielectricbreakdown. Increasing the thickness of the structure 15 to above acertain level also enhances insulation capabilities.

Thus, it is possible to apply a higher voltage across the electrodes 12while using around the same separation width W as a conventional device.Conversely, this means that for the same applied voltage, a higher fieldstrength can be obtained in the vicinity of the substrate 11 by reducingthe distance between the electrodes 12. This is highly advantageous whenthe method of this invention is applied to the fabrication of an MSMphoto-receiving device. With the use of a scanning probe apparatus toprocess the optically nontransparent conductive film 12, the width W ofthe optical window 13 (that is, the distance between the electrodes 12)can be reduced down to little more than ten nanometers, while at thesame time ensuring the high insulation quality of the protectivestructure 15.

FIG. 2 is a cross-sectional view of a photo-receiving device 10Aaccording to the present invention. The photo-receiving device 10Acomprises a light-absorbing substrate 11 of GaAs or other such suitablesemiconductor on which a pair of electrodes 12 is formed, with the spacebetween the electrodes 12 forming an optical window 13 FIG. 1c lightentry. The electrodes 12 are usually of titanium or other such metal,but can also be formed of silicon or another such semiconductor whichhas been made conductive by the introduction of an appropriate impurity.The light-absorbing substrate 11 can be used as the structural substrateof the photo-receiving device 10A, this is not limitative, as theillustrated structure can be formed using a physical support substrateother than the one shown.

In the case of the conventional photo-receiving device 50 shown in FIG.8, the optical window 13 is in the form of a portion of the surfaceexposed to the air. In contrast, the optical window portion of thephoto-receiving device 10A is provided with a protective insulationstructure formed of a suitable insulation material such as titaniumoxide, silicon oxide or silicon nitride. As well as protecting theoptical window by covering the exposed portion of the substrate 11, thisstructure 15 also serves as an optical guide. This optical guide 15needs to at least be light-permeable, that is, transparent orsemitransparent with respect to the light I_(P) to be detected. Theabove are examples of materials that can be treated as transparent tolight of most wavelengths.

Since in this arrangement the portion of the substrate 11 between theelectrodes is not exposed, it eliminates or alleviates the surfacedischarge on the exposed surface of the substrate window 13 that is aproblem with the conventional arrangement, and, using the same width Wbetween electrodes 12 as a conventional configuration, enables a highervoltage to be applied. It is desirable to make the thickness t of theoptical guide 15 greater than the thickness of the electrodes 12. Doingthis increases the creeping distance between the electrodes 12 by anamount corresponding to the increase in thickness, thus providingadequate resistance to the type of air gap discharge across theelectrodes 12 to which the conventional device arrangement is prone.Therefore, while in the case of a conventional device configuration therisk of dielectric breakdown imposes constraints on how close theelectrodes 12 can be brought together, thereby placing an upper limit onthe voltage that can be applied, in the photo-receiving device 10A ofthis invention such constraints are considerably reduced.

As has been described in the foregoing, when the width W of the opticalwindow 13 is made to not exceed the wavelength of the light I_(P) to bedetected, the light I_(P) impinging on the light-absorbing substrate 11appears as an evanescent photoelectric field, the photoelectric fieldstrength is increased just in the vicinity of the surface of thesubstrate 11 and the incident light I_(P) is absorbed in said vicinityof the substrate 11 surface. Since in the inventive photo-receivingdevice 10A a higher voltage can be applied across the electrodes 12 thanin a conventional device, and the electrical field strength is thereforehigher at the surface than inside the substrate, excited carriers(electrons and holes) generated by the incident light I_(P) in thevicinity of the surface of the substrate 11, close to the electrodes 12,are drawn at high speed to the respective electrode (the relativelypositive electrode in the case of electrons, and relatively negativeelectrode in the case of holes). In other words, photoelectricconversion (photodetection) can be effected at a higher speed. Moreover,the high speed at which the carriers are drawn away as a result of thehigh voltage applied means that less recombination takes place, whichhelps to increase the sensitivity and output of the device.

FIG. 3 shows more details of the process used to fabricate the MSMphoto-receiving device 10A of FIG. 2 according to the method of thisinvention. With reference first to FIG. 3 (A) , a full-surfaceconductive layer 12 of titanium is deposited on a GaAs semiconductorsubstrate 11. A STM probe P is positioned near a prescribed surfaceportion of the titanium layer 12, as shown in FIG. 3 (B), and apotential of 5 V is applied between the probe P and the layer 12 underatmospheric conditions (that is, in an environment that includesmoisture), setting up a tunneling current, while at the same time theprobe is scanned in a direction S normal to the drawing sheet. Thescanning speed is set to produce a titanium oxide line 15' having awidth of 100 nm. Considerable control over the width and thickness ofthe line 15' can be exercised by adjusting the voltage and scanningspeed. The width of the line 15' can be increased by also oscillatingthe probe P to each side of the scan line. The titanium line 15' thusformed to a prescribed length constitutes the optical guide 15, and thenon-oxidized portions of the titanium layer at each side form electrodes12. Thus, the fabrication of the electrodes 12 and the optical guide 15between the electrodes 12 is rationalized into a single procedure.

With reference to FIG. 3 (C), mounting electrodes 14 of Ti/Au, forexample, are formed at a desired portion on the electrodes 12 tofacilitate a connection to an external circuit that may be required.With reference to FIG. 3 (D), ground electrodes 14' of Ti/Au, forexample, are formed parallel to a stripe that includes electrodes 12 andoptical guide 15. In this example, stripes are each 5 μm wide and are 5μm apart. This enables a bias line Lb that applies a bias voltage Vb tobe connected to one of the electrodes 12, and a signal line Lr to aresistance load R to be connected to the other electrode 12, and aground line Le to be connected to each of the ground electrodes 14'forming a shield structure.

The measurement system shown in FIG. 4 was used to evaluate thethus-fabricated photo-receiving device 10A, using the electro-opticalsampling method. This is a method in which polarization changes in alaser beam corresponding to changes in the electrical field of anelectro-optical crystal disposed on the circuit to be measured aredetected, producing an electrical signal that is measured at a timeresolution in the femtosecond range. The light source was a collidingpulse mode-locked (CPM) dye laser 22, fed by an argon ion laser 21, withan output of around 10 mW, an output pulse width of 40 fs, and awavelength of 620 nm. The titanium oxide optical guide 15 of thephoto-receiving device 10A exhibited adequate transparency to this lightof wavelength 620 nm, and satisfactory electrical insulation properties.The beam from the CPM dye laser 22 is split by a 9:1 beam splitter 23into an exciting beam I_(P) and a sampling beam Is, respectively. Theexciting beam I_(P) is passed through a variable delay apparatus 32 toadjust the difference with respect to the light path of the samplingbeam Is, and is then directed into the optical guide 15 of thephoto-receiving device 10A. The sampling beam Is is passed through ahalf-wave plate 24 and polarizer 25 to adjust the polarizationdirection, and then into an electro-optical (EO) probe 31. The EO probe31 is a LiTaO₃ plate 300 μm long, 250 μm wide and 50 μm thick, with anelectro-optical coefficient of 35.8 pm/V, and has a multilayer,dielectric antireflection coating on the rear surface of the crystal incontact with the photo-receiving device. The crystal orientation of theEO probe 31 and the polarization direction of the sampling beam Is areset for optimum sensitivity with respect to an electrical fieldperpendicular to the stripe.

The reflected sampling beam Is phase modulated by electrical fieldpermeation into the EO probe 31 is subjected to phase compensation by aBabinet-Soleil compensator 26, deflected by a polarizing beam splitter27 and intensity modulated by a pair of photoreceivers 28a and 28b. Theoutputs of the photoreceivers 28a and 28b are passed through adifferential amplifier 29 to a lock-in amplifier 34 and thereby lockedin to the same 1 MHz that is applied to the fabricated device by asignal generator 33. FIG. 5 displays a plot of the measurement resultsbased on the output of the lock-in amplifier 34. The vertical axis isfield strength and the horizontal axis is time. With measurements takenat a point 70 μm from the photo-receiving device 10A, a full width athalf maximum (FWHM) electrical-pulse value of 570 fs was obtained. At a3 dB region this is equivalent to 790 GHz, which, without dispute, is atthis point world record speed for this type of photoconductivephoto-receiving device.

Thus, the device fabrication of this invention makes it possible todefine the region for the optical window 13 and to form the protectiveinsulation structure (optical guide) 15 for the optical window 13 in asingle process. In addition to this basic effect, it enables the opticalwindow 13 to be formed to a width as fine as around 100 nm, so it doesnot exceed the wavelength of the light to be detected, and the formationof a protective insulation structure 15 that exhibits high quality andhigh dielectric resistance. Applying the method to the fabrication of anMSM photo-receiving device results in a device that combines high speedwith high sensitivity.

Specifically, when the width W of the optical window 13 is no largerthan the wavelength of the light to be detected, the light I_(P)impinging on the light-absorbing substrate 11 appears as an evanescentphotoelectric field, the photoelectric field strength is increased justin the vicinity of the surface of the substrate 11 and the incidentlight is absorbed in this substrate 11 surface vicinity. Because thephoto-receiving device 10A is provided with the high quality protectiveinsulation structure 15, with its high dielectric resistance, comparedto a conventional device a higher voltage can be applied across theelectrodes 12, and the electrical field strength is therefore higher atthe surface than inside the substrate, excited carriers (electrons andholes) generated by the incident light I_(P) in the vicinity of thesurface of the substrate 11, close to the electrodes 12, are drawn athigh speed to the respective electrode (the relatively positiveelectrode in the case of electrons, and relatively negative electrode inthe case of holes). In other words, photoelectric conversion(photodetection) can be effected at a higher speed. Moreover, the highspeed at which the carriers are drawn away as a result of the highvoltage applied means that less recombination takes place, which helpsto increase the sensitivity and output power of the device. Thisinvention provides a device that is faster and more sensitive than aconventional device. This is because even if, in the device of thisinvention, the width W of the gap between the electrodes 12 is increasedto more than the 300 nm of a conventional device, while observing theprovision that the width W does not exceed the wavelength of the lightto be detected, the lower risk of a dielectric breakdown occurring meansthat a higher voltage can be applied across the electrodes 12. Doingthis would ease the fabrication process burden. An example is theabove-described processing method in which a scanning probe apparatus isapplied to the formation of the protective insulation structure 15; forexample, forming an oxide line of a desired width by oscillating theprobe P to each side of the scan line, as described above. Instead, itwould be possible to use a different existing fine pattern processtechnology, such as electron beam lithography or selective epitaxis.

Modifications of the invention will now be described. First, theprotective insulation structure/optical guide 15 does not have to be aninsulator. It is only required that it be located between the electrodes12 and exhibit higher resistance than that of the substrate 11, and assuch may be formed of a semiconductor based on an insulation material.If GaAs is used for the substrate 11, for light-permeability (minimalphoto-absorption) the structure 15 could be formed of AlAs, GaP or othersuch semiconductor having a larger band-gap than that of the GaAs.However, when such a semiconductor is used to form the optical guide 15,to ensure complete insulation a small gap should be left between theguide 15 and at least one of the electrodes 12 and this gap filled withan insulation material. This corresponds to an embodiment in which theoptical guide 15 is constituted of materials having differentproperties.

FIG. 6 shows an embodiment of a photo-receiving device 10B in which theoptical window is provided with two or more discernible optical guides15 (only two are shown in the drawing), and a nontransparentinterstitial portion or means 12' is provided between adjacent guides15. The detection sensitivity can be increased by the number of opticalguides 15 having a width not exceeding the wavelength of the light to bedetected. In principle the interstitial means 12' may be conductive orinsulative, but preferably are conductive, since that enables them to beformed of the same material as the film used to form the electrodes 12at each side. Any desired fine pattern process technology may be used toform the optical guides 15. A scanning probe based process is highlyrational, as it enables the process used to form the guides 15 to beutilized to also form the electrodes 12 and interstitial means 12', allat the same time.

As has already been mentioned, it is possible for the optical guides 15to be formed of a mixed multiplicity of materials having differentproperties, such as insulators and semiconductors, as long as this doesnot depart from the defined scope of the invention. Moreover, theoptical guides 15 may include a structure that enables the opticalproperties of the guides to be varied. Such a facility is included inthe device 10C shown in FIG. 7. In this arrangement, part of the opticalguide 15 formed in the optical window has a multilayer structurecomprising, from the side in contact with the substrate 11, an n-type(or p-type) semiconductor layer 37, a GaAs/AlGaAs or InAs/InGaAssuperlattice layer 39, and a p-type (or n-type) semiconductor layer 38.On top of this is a nontransparent protective insulation structure 15"formed of a single insulation or semiconductor material. For externalcontrol of optical properties, a contact electrode 36 of a metal orsemiconductor substance is provided that extends down through thestructure 15" to form an electrical contact with the semiconductor layer38. The semiconductor layer 37 is also provided with a controlelectrode. In this example, one of the electrodes 12 (the one on theleft of the drawing) also functions as this contact electrode.

With this arrangement, an electrical field can be applied to thesuperlattice layer 39 by applying an external voltage between one of theelectrodes 12 and the control electrode 36. The quantum containmentStark effect corresponding to the strength of the electrical fieldchanges the absorption coefficient and refractive index with respect towavelengths in the vicinity of the ends of the bands, making it possibleto control the absorption peak of the excitons, the appearance of whichis a characteristic feature of quantum structures. The spectrum of theincident light I_(P) can therefore be established from changes in theabsorption coefficient for each wavelength, which can be used to improvethe functional quality and range of a photo-receiving device. AFabry-Perot resonator structure can be incorporated in the optical guide15 to provide a photo-receiving device with wavelength selectivity, asthe resonance wavelength can then be controlled by using the applicationof an outside voltage to electrodes 12 and 36 to control the refractiveindex. Such an arrangement could be envisaged, with reference to FIG. 7,by viewing the superlattice layer 39 as the requisite internal waveguideof the resonator and the upper and lower layers 37 and 38 as being bothreflectors and refractivity control electrodes. In accordance with thisinvention, such an embodiment also includes other structures forcontrolling optical properties.

In the configuration shown in FIG. 7, an insulation layer 40 is disposedbetween the electrode 12 on the right and the optical guide 15 thatincludes the superlattice layer 39, to prevent the semiconductor layer37 from causing an electrical short between the electrodes 12. Althougha gap may be used for this instead of the insulation layer 40, providingthe layer 40 is not complex, since it can be formed at the same time asthe insulation structure 15".

Other modifications and variations of the present invention clearly arealso possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

What is claimed is:
 1. A method of fabricating a photo-device,comprising a step of forming an optically nontransparent, electricallyconductive layer on a surface of a photoelectric conversion substrate,and a step whereby a portion of the optically nontransparent,electrically conductive layer is transformed by a scanning probe processapparatus to form, over an optical window, a light-permeable protectiveinsulation structure of a prescribed width and length, and at the sametime define a region of the optical window on the substrate surfacebeneath the light-permeable protective insulation structure.
 2. Afabrication method according to claim 1, wherein the opticallynontransparent, electrically conductive layer is of a metal or an alloy.3. A fabrication method according to claim 1, wherein the opticallynontransparent, electrically conductive layer is a semiconductor.
 4. Afabrication method according to claim 1, wherein the opticallynontransparent, electrically conductive layer includes a semiconductorsuperlattice structure.
 5. A fabrication method according to claim 1,wherein the transformation is oxidation.
 6. A fabrication methodaccording to claim 1, wherein the portion of the opticallynontransparent, electrically conductive layer that is transformed istransformed in multiple mutually separated locations on said layer.
 7. Afabrication method according to claim 1, wherein the photoelectricconversion substrate is a light-absorbing substrate with aphoto-receiving function,non-transformed portions of the nontransparent,electrically conductive layer function as electrodes for applying avoltage to the photoelectric conversion substrate, and thelight-permeable protective insulation structure functions as an opticalguide for directing light into the light-absorbing substrate.